This commit makes it well defined as to whether or not the output
buffer passed in to the data callback is initialized to zero or left
undefined. By default it's initialized to zero, but can be changed to
undefined by setting noPreZeroedOutputBuffer in the device config.

* ma_rb_reset()
* ma_rb_available_read()
* ma_rb_available_write()
* ma_pcm_rb_reset()
* ma_pcm_rb_available_read()
* ma_pcm_rb_available_write()

Fix a few tiny memory leaks

This happens when stopping the device while nothing is being played and
is due to how WASAPI does not signal an event in loopback mode when
nothing is being played.